It is well known that luminous elements of the three colours red, green and blue have a different response time. Response time is the time, which the luminous element takes to react to a given input. The differences in the response time are the reason that a coloured trail/edge appears as an artificial colour behind an edge and at the front edge of a white object moving in front of a black background on a display device. Luminous elements of different colour like the three phosphors of a plasma display have different chemical properties and are different concerning the response time, which generates artifacts as coloured trails and edges especially visible at moving black/white edges due to the so-called phosphor lag. In order to reduce the disturbing character of such coloured trails/edges, it already has been recommended to detected moving edges and to add an artificial trail behind these edges in order to discolour it. However, such solutions require a significant amount of line memories, which make it expensive, and require the use an edge detector which is typical sensitive to noise. Furthermore, it has been recommended to shift sub-fields of the three colour components differently whereby a moving vector indicating moving objects between two successive images is used to displace sub-fields dependent on movement and type of the phosphor. However, a moving vector is required and sub-field shifting is limited to the pixel resolution. Therefore, it already has been recommended to use different groups of sub-fields for displaying a video image for at least two types of phosphor. It reduces the length of the colour trail; however, it does not handle its discoloration. Already long time ago, it has been recommended to correct differences in persistence of the phosphors in a colour image display screen by detecting transitions between a first grey level and an adjacent second grey level and if the transition is greater than a threshold to force the state of the cell covered with a persistent phosphor to the second grey level before the end of the frame period, see e.g. U.S. Pat. No. 6,377,232. Furthermore, anti-motion blur filtering of the input video signal has been recommended. As the viewer of moving objects on a matrix display integrates the intensity of the pixels along the motion trajectory, motion blur may be reduced by enhancing high spatial frequencies of moving objects, which cause noise enhancement and noise modulation. Therefore, said anti-motion blur filtering is also based on detecting edge characteristics in each image of the input video signal.
Invention
It is an aspect of the invention to provide an arrangement and a method for processing video signals to be displayed on a display device having at least two kinds of luminous elements with different response time with less expenditure and high efficiency in reducing phosphor lag artifacts at edges of moving objects suffering from phosphor lag artifacts on display devices.
It is a further aspect of the invention to make it possible to remedy afterglow defects of moving objects on display devices, such as coloured edges or trails at black-white or white-black transitions to reduce phosphor lag artifacts on display panels without edge detection of the moving object, without a significant amount of line memories and with improved noise performance.
This object is achieved by means of the features specified in independent claims. Advantageous designs and developments are specified in dependent claims.
Although less effective edge detection shall not be used to discolour edges of a moving object suffering from the phosphor lag effect, a specific low-pass filtering of the colour component of the video signal, which corresponds to the fastest luminous element of the display device, is recommended. Said specific low-pass filtering of the colour component of the video signal corresponding to the fastest luminous element is performed dependent on the horizontal speed of the moving object although of course an object may move in all directions on a display panel. That means that the present invention starts from the assumption that phosphor lag occurs only in horizontal direction, which is of course not true as of course an object may move in all directions and not all contours of a moving object suffering from said phosphor lag effect as it will be shown in a detailed description of embodiments below. However, although there are said contradictions, a horizontal spatial equalisation of the response time of luminous elements with different response time is recommended.
It is well known that a motion estimator is able to provide motion vector signals representing the movement of an object on the display panel. Motion vector signals or data representing the movement of an object on the display panel may be generated from the video signal or are already available in the video signal. Said motion vectors are representative for the moving speed of an object to be displayed on the display device in horizontal and vertical direction. Therefore, in a first glance it could be assumed that it would be sufficient to use the horizontal motion vector for phosphor lag compensation. However, there are also edges of a moving object, which do not suffer from the phosphor lag effect. Therefore, a horizontal speed correction unit is recommended which reduces the horizontal motion vector in case that the moving object has also a vertical moving component. Furthermore, for said specific low-pass filtering the use of a FIR-filter is recommended, wherein the filter coefficients are determined pixel-by-pixel according to the horizontal speed magnitude of a central pixel. That means that for each speed one set of coefficients is determined, which is calculated according to equations and conditions shown below. The function of the FIR filter is to emulate a trailing trail following moving edges at least for the colour component in the video signal, which corresponds to the fastest luminous element of the display panel. According to an exemplary embodiment, wherein blue luminous elements of the display device are the fastest, which means have a response time shorter than the response time of red and green luminous elements, which both have nearly the same response time, it is sufficient to apply said specific low-pass filtering with the recommended FIR-filter only for the blue channel. As the FIR filter has to emulate a trailing trail following moving edges for each speed, the set of coefficients is always one sided: for left-to-right motion speeds, only the coefficients that apply to the centre and right pixels are different from zero; and for right-to-left motion speeds, only the coefficients that apply to the centre and left pixels are different from zero. The reason why for left-to-right motions the right pixels are non-zero is that for a given center pixel the trail occurs when the moving object has passed by, and is therefore already located to the right of the center pixel.
The FIR filter coefficients are obtained by applying the following equations and conditions:
            a      0        =          1      -              exp        ⁡                  (                      -                          α                                              v                                                              )                                        a                  n          +          1                    =                        a          n                *                  exp          ⁡                      (                          -                              α                                                    v                                                                        )                                ,  wherein a0 denotes a centre pixel coefficient—the current pixel, and an denotes either the left or the right coefficients dependent on the moving direction. |v| denotes the speed amplitude in pixels per frame, and α is a constant that denotes the magnitude of the required correction for a specific panel technology.
Furthermore, for all values of v and α it shall be ensured that
                    ∑                  n          =          0                ∞            ⁢              a        n              =    1    ,which means, that for a specific speed, the sum of all coefficients shall be always equal to 1. For cases, where the sum of coefficients is different from one, the coefficients are multiplied with a constant value to ensure that the filter gain is always equal to 1. Said filter gain of the FIR filter equal to 1 has to be ensured so that an input video flat field is not modified by the FIR filter. The above-mentioned constant α, which denotes the magnitude of the required correction, is a function of the technology for which the recommended method and arrangement shall be applied. The constant α has to be experimentally discovered for a best result and is dependent on the video frame rate and the response time of the luminous elements, which shall be equalized. For a plasma display panel technology, where the response time of luminous elements of the blue colour component is much shorter in comparison to red and green, which have about the same response time, a value of about four has been discovered as it will be shown in an exemplary embodiment. This is a typical example for plasma display panel, however, the recommended arrangement and method are also applicable to display panel of other technologies, wherein luminous elements with different response time are used, the colour of the fastest luminous element is different to blue or several luminous elements have different response time.
As shown above, a horizontal spatial equalisation of the response time of luminous elements with different response time is recommended to reduce phosphor lag artifacts on display devices by providing a sliding average value of at least one color component signal of the video signal, which represents luminous elements with a shorter response time, dependent on a horizontal speed signal generated from a horizontal motion vector signal attenuated by a vertical motion vector signal. Said horizontal and vertical motion vector signals represent the speed of an object moving on the display device. The horizontal motion vector signal is attenuated by a vertical motion vector signal to ensure that phosphor delay compensation will be manly applied to edges of horizontally moving objects. Providing a sliding average value of at least one color component signal of the video signal means that a sum of a certain number of successive video signal values is generated, which is divided by said number of values and that said group of values is continuously shifted by one value and a new average value is formed. Result of said filtering are filtered values similar to a low-pass behavior of the color component signal of the video signal, which controls the luminous elements having the shorter response time.
It has been found that low-pass filtering the colour component signal of a video signal, which corresponds to the fastest colour component of a display device, dependent on a horizontal moving speed signal generated from motion vectors of a moving object to be displayed on the display device is an efficient method to reduce phosphor artifacts and to discolour edges or trails at black-white or white-black transitions of the moving object to be displayed on the display device. The recommended method and a device to perform said method require little expenditure by realisation with a horizontal working FIR filter. Line memories and an edge detector are not needed, so that a high expenditure for phosphor lag compensation is avoided. Furthermore, the recommended phosphor lag compensation is less sensitive to noise and requires little expenditure as the compensation is performed in horizontal direction and in case of most plasma displays, for one colour channel only. It is applicable for any display technology where the time response of different colour elements of the display is not the same for all colours.